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168 PBINCIPLES OF' CHEMISTBY ...

As a hydrate, orthophospborio acid should be expressed, after the
fashion of other hydrates, as containing three water residues (hydroxyl
groups), i.e. as PO(OH)₃. This method of expression indicates that
the type PX_S, seen in PH₄I, is here preserved, with the substitu*
tion of X_a by oxygen and X₃ by three hydroxyl groups. The same
type appears in P001₃, PC1₆, PF_6> &c. And if. we recognise phos*
phoric acid as PO(OH)₈, we should expect to find three anhydrides
corresponding with it: (1) [PO(OH)₂]_aO, in which two of the three*
hydroxyls are preserved; this is pyrophosphoric acid, H₄P₈07. (2)
PO(OH)0, where only one hydroxyl is preserved. - This is metaphos*
phoric acid. (3) (P0)_a0₃ or P_aO_g, that is, perfect phosphoric anhydride.
Therefore, pyro* and meta-phosphoric acids are imperfect anhydrides (or
anhydro-acids) of orthophoepKoric acid,¹⁹

feeble aoida as acetic-and eulphurous, and even in water containing carbonic acid. Th»

latter fact is of immense importance in nature, since by reason of it rain water is able to

transfer the calcium phosphates in the soil into solutions which are absorbed by planta.

The solubility of the normal salt in acids takes place by virtue of the formation of an

acid salt, which, is evident from the quantity of acid required for its solution, and xnoro

especially from the fact that the add solutions when evaporated give crystalline scale*

of the acid calcium phosphate, CaJI^PO^a, soluble in water. This solubility of the aci65

ealt forms the basis of the treatment by acids of bones, phosphorites, guano-, and other

natural products containing the normal salt and employed for fertilising the soil. Thfl> .

perfect decomposition requires at least 2HjS0₄ to CagfPO^Jj, but in reality less is taken,,

BO that only a portion of the normal salt is converted into the acid salt. Hydrochloric/

acid is sometimes used, (In practice such mixtures are known as superphosphates),

Certain experiments, however, show, that a thorough grinding, the presence of organic, and

especially of nitrogenous, substances, and the porous structure of some calcium phosphates,

(for example, in burnt bones), render the treatment of phosphoric manures by acids super*

fiuous—thai is, the crop is not improved by it.

¹⁹ In this sense the ortho-acid itself might be regarded as an anhydro-aoid, counting*
P(HO)s as the perfect hydrate, if PHs existed; but as in general the normal hydrate.9
correspond with the existing hydrogen compounds with the addition of up to 4 atoms ol
oxygen, therefore PH₅0₄ is the normal acid, just as SE^ and C1H0₄ j while NHO»
CHjOj are meta-aoids, or higher normal acids (NE$0₄ and CH₄0₄) with the loss of a
molecule of water,

In order to see the relation between the ortho-, pyro-, and meta-phosphorio-acids, the>
first thing to remark in them is that the anhydride P₄0s is combined with 8, 2, and I
molecules of water. In the absence of data for the molecular weight of ortho- and pyro«i
phosphoric acids it is necessary to mention that all existing data for meta-phoaphorio acid
indicate (Note 21) that its molecule is much more complex and contains at least HjP₈O&
or HflP₆O₁₈. The explanation of the problems which here present themselves can, it seems
to m6, be only looked for after a detailed study of the phenomena of the polymerisations!
of mineral substances, and of those complex acids, such as phosphomolybdio, which wo
shall hereafter describe (Chapter XXI) A similar instance is exhibited in the solubility]
of hydrate of silica, (produced by the action of silicon fluoride on water) in fused raefa*
phosphoric acid, with the formation, on cooling, of an octahedral compound (sp. gr., 8*1|
containing SiO^PaOs. A certain indication (but no proof) that ordinary orthophos*
phoric acid ik polymerised is given by Standenmaier (1898), who obtained a salt,
by the action of a solution of KttjPO₄ upon KgC0_s; and a compound,
, corresponding to the doubled molecule of HsP0_4l by the action of KE^PO*.



	168 PBINCIPLES OF' CHEMISTBY ... As a hydrate, orthophospborio acid should be expressed, after the fashion of other hydrates, as containing three water residues (hydroxyl groups), i.e. as PO(OH)₃. This method of expression indicates that the type PX_S, seen in PH₄I, is here preserved, with the substitu* tion of X_a by oxygen and X₃ by three hydroxyl groups. The same type appears in P001₃, PC1₆, PF_6> &c. And if. we recognise phos* phoric acid as PO(OH)₈, we should expect to find three anhydrides corresponding with it: (1) [PO(OH)₂]_aO, in which two of the three* hydroxyls are preserved; this is pyrophosphoric acid, H₄P₈07. (2) PO(OH)0, where only one hydroxyl is preserved. - This is metaphos* phoric acid. (3) (P0)_a0₃ or P_aO_g, that is, perfect phosphoric anhydride. Therefore, pyro and meta-phosphoric acids are imperfect anhydrides* (or anhydro-acids) of orthophoepKoric acid,¹⁹ feeble aoida as acetic-and eulphurous, and even in water containing carbonic acid. Th» latter fact is of immense importance in nature, since by reason of it rain water is able to transfer the calcium phosphates in the soil into solutions which are absorbed by planta. The solubility of the normal salt in acids takes place by virtue of the formation of an acid salt, which, is evident from the quantity of acid required for its solution, and xnoro especially from the fact that the add solutions when evaporated give crystalline scale* of the acid calcium phosphate, CaJI^PO^a, soluble in water. This solubility of the aci65 ealt forms the basis of the treatment by acids of bones, phosphorites, guano-, and other natural products containing the normal salt and employed for fertilising the soil. Thfl> . perfect decomposition requires at least 2HjS0₄ to CagfPO^Jj, but in reality less is taken,, BO that only a portion of the normal salt is converted into the acid salt. Hydrochloric/ acid is sometimes used, (In practice such mixtures are known as superphosphates), Certain experiments, however, show, that a thorough grinding, the presence of organic, and especially of nitrogenous, substances, and the porous structure of some calcium phosphates, (for example, in burnt bones), render the treatment of phosphoric manures by acids super* fiuous—thai is, the crop is not improved by it. ¹⁹ In this sense the ortho-acid itself might be regarded as an anhydro-aoid, counting* P(HO)s as the perfect hydrate, if PHs existed; but as in general the normal hydrate.9 correspond with the existing hydrogen compounds with the addition of up to 4 atoms ol oxygen, therefore PH₅0₄ is the normal acid, just as SE^ and C1H0₄ j while NHO» CHjOj are meta-aoids, or higher normal acids (NE$0₄ and CH₄0₄) with the loss of a molecule of water, In order to see the relation between the ortho-, pyro-, and meta-phosphorio-acids, the> first thing to remark in them is that the anhydride P₄0s is combined with 8, 2, and I molecules of water. In the absence of data for the molecular weight of ortho- and pyro«i phosphoric acids it is necessary to mention that all existing data for meta-phoaphorio acid indicate (Note 21) that its molecule is much more complex and contains at least HjP₈O& or HflP₆O₁₈. The explanation of the problems which here present themselves can, it seems to m6, be only looked for after a detailed study of the phenomena of the polymerisations! of mineral substances, and of those complex acids, such as phosphomolybdio, which wo shall hereafter describe (Chapter XXI) A similar instance is exhibited in the solubility] of hydrate of silica, (produced by the action of silicon fluoride on water) in fused raefa* phosphoric acid, with the formation, on cooling, of an octahedral compound (sp. gr., 81\| containing SiO^PaOs. A certain indication (but no proof) that ordinary orthophos phoric acid ik polymerised is given by Standenmaier (1898), who obtained a salt, by the action of a solution of KttjPO₄ upon KgC0_s; and a compound, , corresponding to the doubled molecule of HsP0_4l by the action of KE^PO*.